Diffusers are well known in the art. A diffuser is a device for reducing the velocity and increasing the static pressure of a fluid passing through the same such as in an automotive exhaust system. Diffusers of this type have an inlet cross-sectional flow area that is less than their outlet cross-sectional flow area. Often, diffusers are used for the specific purpose of reducing fluid velocity or increasing fluid pressure. However, the use of a diffuser may be dictated simply by the physical requirements of the exhaust system such as increasing the cross-sectional flow area of a passage in order to connect sections of different cross-sectional area. This is the situation wherein a small diameter exhaust pipe is connected to a larger diameter housing for a catalytic converter. However, such diffusers which are dictated by the physical connection between pipes of different sizes do not necessarily allow for accurate control of fluid velocity and pressure to achieve optimum performance of an automotive combustion engine or its catalytic converter.
An automotive catalytic converter is used to decrease exhaust emissions by converting carbon monoxide, nitrogen oxides, and unburned hydrocarbons to more innocuous compounds. The catalytic converter includes a substrate which carries a catalyst for converting the exhaust emissions. Suitable catalysts are known to those skilled in the art and include for example platinum-containing compositions. For platinum-containing compositions or compositions including other expensive metals it is important to utilize the catalytic converter in the most efficient manner. Thus, it is important to adjust the flow characteristics to provide a uniform velocity distribution across the substrate cross-section so that the expensive catalyst can convert the emissions while utilizing the smallest amount of catalyst carried on the catalytic converter substrate.
The uniform velocity field at the substrate inlet face is also desirable because it leads to the smallest pressure loss across the converter for a given substrate geometry and engine flow. High pressure loss leads directly to increased engine back pressure which results in reduced engine power at high load and reduced fuel economy.
The catalytic converter substrate usually is carried by a housing having a cross-sectional area substantially greater than that of the exhaust pipe running from the combustion engine to the catalytic converter. Accordingly, the catalytic converters are often built with a converter inlet snorkel. The snorkel has a larger cross-sectional area at one end where it is connected to the housing surrounding the catalytic converter substrate and a smaller cross-sectional area at the other end where the snorkel is connected to the exhaust piping coming from the automotive engine. The snorkel may have a variety of shapes such that a cross-section may be circular, elliptical, rectangular or otherwise. Although the catalytic converter snorkel in a very broad sense is a diffuser, the dimensions of the snorkel are defined solely by the physical requirements of connecting the larger cross-section housing for the catalytic converter to the smaller diameter exhaust pipe leading from the automotive engine. Thus, the snorkel does not necessarily define a diffuser which will meet the objectives of providing a sufficiently uniform converter-inlet velocity distribution (and hence the largest and most uniform residence time) to convert the exhaust emissions utilizing the smallest amount of expensive metal catalyst carried on the ceramic substrate.
Single-wall diffusers which connect from the exhaust pipe to the outer shell of the catalytic converter housing often suffer from thermal fatigue and creep. Hot exhaust gases directly contact the diffuser where it makes mechanical connection to the catalytic converter housing and the exhaust pipe. The thermal cycling associated with this portion of the diffuser results in thermal stresses on the connections and results in reduced durability of such an exhaust system. Further, a single-wall diffuser conducts heat to the catalytic converter housing which degrades the performance and durability of the mounting insulation between the housing and the ceramic substrate.
The present invention overcomes the deficiencies of the prior art by providing a two-wall diffuser integral cast construction as described below.